More on LANs Module C Copyright 2001 Prentice Hall.
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Transcript of More on LANs Module C Copyright 2001 Prentice Hall.
More on LANsModule C
Copyright 2001 Prentice Hall
More on LANs• More on Ethernet
– 802.3 10Base5 and 10Base2– 100Base-TX signaling– Ethernet II versus 802.3– 802.2 Logical Link Control Layer Frame
• More on 802.5 Token-Ring Networks• 802.11 Wireless LANs• Bridges versus Switches
Ethernet Recap• Created by Digital Equipment Corporation, Intel,
and Xerox• Taken over by the 802.3 Working Group of the
IEEE 802 LAN MAN Standards Committee• 802.3 has since produced many standards
– Especially at the physical layer: 10Base5, 10Base2, 10Base-T, 100Base-TX, 1000Base-X
– Only one MAC layer standard--the 802.3 MAC layer standard for framing and CSMA/CD
Original Ethernet Versus 802.3 10Base5
• Created by Digital, Intel, and Xerox– Popular version was Ethernet II
• 802.3 Working Group took over– Created the first Ethernet standard, 802.3
– Slightly changed the physical layer; became 10Base-5
– Changed the data link layer (later)
• 10Base5– 10 Mbps, baseband, 500 meters/segment
Ethernet 10Base5
• Multidrop Layout– Main trunk cable: thick coaxial cable
• Up to 500 meters long
– Drop cable: 15-wire cable• Up to 50 meters
• Also called the attachment unit interface (AUI)
TrunkCable
DropCable(AUI)
Up to 500 meters
Ethernet 10Base5
NIC
15-pinAUI Connector
Drop Cable(Attachment UnitInterface or AUI)
15 wires50 m maximum
Trunk CableCoaxial Cable
500 m maximum
N-Connector
Transceiver (Medium Attachment Unit)
Ethernet 10Base5
1. Sending NIC Transmits a bit
2. TransceiverBroadcasts the bit
3. Each TransceiverSends the bit to
Its NIC
Ethernet 10Base5
• Broadcast (Bus) Topology
– Station B transmits
– Signal travels down trunk to all other stations
A B C
Ethernet 10Base-5
• Up to 5 segments connected by repeaters– 2,500 meters maximum distance between
farthest stations– Four repeaters maximum– No loops
Repeater RepeaterDrop cable
Segment of Trunk Cable AUI Port
Ethernet 10Base2 (802.3a)• Cheaper Physical Layer Standard
– NICs have BNC connector– T-connector attaches to it– T-connector has BNC connectors for cable runs
attaching it to adjacent stations
NIC
BNC T-connectorTo next
NICTo next
NIC
Ethernet 10Base2 (802.3a)• Segments are thin coaxial cable
– Run only between NICs– Daisy chain of NICs is a segment– Terminator at end of each segment– Up to 30 stations per segment– 5 segments (4 repeaters) maximum– 10Base2: 185 meters/segment
NIC NIC NICTerminator
Repeaters can Mix 802.3 PHY Standards
Repeater Repeater
NICBNC
AUIConnectors
RJ-45
UTP DropCables
10Base5 Trunk Cable
10Base2
10Base-T
Repeaters must have the correct ports
Ethernet II vs 802.3 Frames• Ethernet II Frame
– Ethertype field (2 bytes)– Tells protocol of message in data field (e.g., IP)– No length field– No 802.2 LLC layer; Full data link layer protocol
Preamble SFD DA SA Ethertype
Preamble SFD DA SA Length LLC frame FCS
Data FCS
Ethernet II Frame
802.3 Frame
Ethernet II vs 802.3 Frames• Why the Differences
– Lack of a length field could cause problems
– 802 would create multiple standards; Giving them all the same connection to the next higher layer (802.2 seemed wise)
Preamble SFD DA SA Ethertype
Preamble SFD DA SA Length LLC Frame FCS
Data FCS
Ethernet II Frame
802.3 Frame
Ethernet II vs 802.3 Frames• Telling them Apart
– NIC must recognize incoming frame’s type
– If 2 bytes after SA <= 1500, must be 802.3 because of maximum length limitation in 802.3
– If 2 bytes after SA >1500, it’s an Ethertype
Preamble SFD DA SA Ethertype
Preamble SFD DA SA Length LLC Frame FCS
Data FCS
Ethernet II Frame
802.3 Frame
802.2 LLC Layer Functions• Logical Link Control Layer (LLC)
– Links subnet to next-higher-layer (internet)– One LLC standard (802.2) for all 802 LANs– Makes interface to internet layer simple
Internet Layer
802.2 LLC
802.3 MAC
802.3 10Base-T
Internet Layer
802.2 LLC
802.3 MAC
802.3 100Base-T
Internet Layer
802.2 LLC
802.5 MAC
802.5 16 Mbps
802.2 LLC Layer Functions
• Logical Link Control
– At the MAC layer, the destination NIC discards incorrect MAC layer frames
– 802.2 standard provides optional error correction to retransmit lost or discarded LLC frames
802.2 LLC Frame• Fields
– Destination Service Access Point (DSAP) designates the next-higher layer protocol on the receiving device (IP, IPX, etc.)
– There can be multiple internet layer programs on the destination machine; DSAP designates the specific one to get the frame’s data field
– SSAP: Source Service Access Point
DSAPSSAPControlData Field
802.2 LLC Frame
• Fields
– Control field contains instructions to the receiver’s LLC process on the NIC
– Data field contains message of the next higher layer (usually internet)
DSAPSSAPControlData Field
802.2 LLC Frame• Subnet Access Protocol (SNAP) Alternative
for IP and other TCP/IP internet layer standards
– DSAP = AA hex
– SSAP = AA hex
– Control = 03 hex
DSAP=AA
SSAP=AA
OrgCode
Data FieldEther-type
SNAP message
Control=03
802.2 LLC Frame• Subnet Access Protocol (SNAP) Alternative
for IP and other TCP/IP internet layer standards
– Org code lists the organization maintaining a list of protocol codes for next higher layer
– Ethertype (Ethernet type) code names a specific protocol for the data field
DSAP=AA
SSAP=AA
OrgCode
Data FieldEther-type
SNAP message
Control=03
802.2 LLC Frame
• Subnet Access Protocol (SNAP) Alternative for IP and other TCP/IP internet layer standards
– Data field has message of that protocol
DSAP=AA
SSAP=AA
OrgCode
Data FieldEther-type
SNAP message
Control=03